Measuring a rotational speed, which is required in the art on many machines and systems, is physically equivalent to measuring the angular speed. It is generally known to measure the rotational speed of shafts by applying a periodic pattern known as an “encoder” (e.g. a gear) around the circumference of the shaft, which is sensed by a sensor mounted in a fixed position beside the shaft. The sensor is capable of distinguishing between tooth and tooth gap (or other periodically varying properties, such as e.g. magnetic field direction or optical transparency). The sensor then generates an output signal which has the same periodicity as the sensed pattern.
A sensor of this type outputs different signals depending on the signal processing that is present: there are approximately sinusoidal signals, which are usually generated directly from the primary sensor element, or square-wave signals, which are usually generated by comparators by the downstream signal processing. The sinusoidal signals often occur as a sine/cosine signal pair, because this combination has advantages, including with regard to detecting the direction. Any combinations of said signals are possible, so that up to four outputs (and any subset thereof) may exist: sine, cosine, square-wave in phase with sine, square-wave in phase with cosine. All of these signals are analog frequency signals, i.e. the frequency varies continuously within the interval defined by the application. The electrical output signal is a direct image of the encoder. Thus even the square-wave signals cannot be considered to be “digital”, because the discrete amplitude variable does not contain discretized information from the sensor. Depending on the implementation, there is also the possibility that the physical sensor-based process generates a signal at twice the frequency of the encoder pattern. This is the case, for instance, for certain AMR sensor elements, which have an electrical signal period that encompasses only a 180° rotation of the (encoder) magnetic field.
If sensors of the type described are used in measuring devices or control systems, then it must be taken into account that the described output signals in no way complete the measurement process: whereas most measurement systems provide output signals that are either digitally encoded or have an output value that is a direct measure of the measurand, for the sensors considered here, the measured value must first be calculated from the output waveform or pulse sequence. This applies to speed, angular-speed and rotational-speed measurements and also to angle measurements, because often absolute measurements are needed, which are obtained from the periodic signal by counting. The sine/cosine signals here have the advantage of the possibility of interpolation, but do involve a higher degree of analysis effort.
The technology presented here is used in the same way for linear position and speed measurement, but using linear rather than annular encoders.
The delay in the frequency measurement, which results from the fact that the resolution of the frequency measurement is linked to the counter value, causes problems for control processes. For high-resolution measurements, a large number of pulses must be counted. Since counting the frequency only provides the average value of the frequency in the measurement interval, the bandwidth of the measurement is also limited.
In particular when using square-wave signals, which are very common in some applications, e.g. in the automobile, the accuracy is limited by noise. Unlike sinusoidal signals, for which at least some of the noise lies far outside the frequency band of the sinusoidal waveform and hence can be filtered, square-wave signals provide no opportunity for spectral separation of the components. The noise manifests itself as “jitter”, which is the random fluctuation in the position of the pulse edges in the time domain caused by phase noise. A stable square-wave signal requires a very high signal-to-noise ratio in the primary signal of the sensor element. A reduction in this ratio may produce errors in the form of additional, noise-induced pulses, even if the power of the signal is considerably greater than that of the noise.